1. Field of the Invention
The invention relates to a method for magnetic resonance imaging of a body placed in a stationary and substantially homogeneous main magnetic field, the method including at least one measuring sequence comprising the application of an excitation radio-frequency pulse (RF-pulse) for excitation of nuclear dipole moments in at least a portion of the body, the application of at least one refocusing RF-pulse following said excitation RF-pulse and switched gradient magnetic fields for generating position dependent magnetic resonance signals in the excited portion. Such a sequence of RF-pulses is known as a spin-echo sequence and generates one or more nuclear magnetic resonance echo signals (NMR-signals) following the refocusing RF-pulses. The invention also relates to an apparatus for magnetic resonance imaging using such a method.
2. Description of the Related Art
Such method for imaging is known from EP-A 0 210 562. In the known method switched gradient magnetic fields are applied for encoding position information in the NMR-signals. The gradient fields provide slice selection, phase encoding and read gradients. As described in the cited EP-A 0 210 562, switching of the gradients causes eddy currents in the apparatus, which eddy currents generate magnetic fields of their own and disturb the applied magnetic fields. The disturbed magnetic fields lead to artefacts in a reconstructed image, which artefacts may exhibit themselves as local low level signal, local absence of signal in an image or the presence of ghosts in the image. In the known method the influence of eddy currents is compensated for by applying a cancelation gradient magnetic field pulse of a polarity opposite to that of the gradient pulse to be compensated. The applied cancellation gradient magnetic field pulses are constant or approximate the decaying eddy current magnetic fields, at least during data acquisition.
In the known method gradient magnetic fields are added to a measuring sequence in a way that the total time-integrated strength, or gradient area, in an interval between RF-pulses is changed. As these correction fields generate eddy currents themselves, and because of the change in gradient area, the known method is not appropriate for a spin-echo sequence in which a plurality of magnetic resonance signals is generated by repeated application of refocusing RF-pulses following a single excitation RF-pulse. The Carr-Purcell-Meiboom-Gill (CPMG) sequence is such a sequence. A CPMG sequence requires that the phase of a precessing nuclear dipole moment is the same at each refocussing RF-pulse, this condition of equal phase is referred to as the CPMG condition.